An organic light-emitting diode device, also called an OLED device, commonly includes a substrate, an anode, a hole-transporting layer made of an organic compound, an organic luminescent layer with suitable dopants, an organic electron-transporting layer, and a cathode. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing, and capability for full-color flat emission displays. Tang et al. described this multilayer OLED device in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
A common problem with OLED displays is sensitivity to moisture. Typical electronic devices require humidity levels in a range of about 2500 to below 5000 parts per million (ppm) to prevent premature degradation of device performance within a specified operating and/or storage life of the device. Control of the environment to this range of humidity levels within a packaged device is typically achieved by encapsulating the device or by sealing the device and a desiccant within a cover. Desiccants such as, for example, molecular sieve materials, silica gel materials, and materials commonly referred to as Drierite materials, are used to maintain the humidity level within the above range. Particular highly moisture-sensitive electronic devices, for example, organic light-emitting devices (OLED) or panels, require humidity control to levels below about 1000 ppm and some require humidity control below even 100 ppm. Such low levels are not achievable with desiccants of silica gel materials and of Drierite materials. Molecular sieve materials can achieve humidity levels below 1000 ppm within an enclosure if dried at a relatively high temperature. However, molecular sieve materials have a relatively low moisture capacity at humidity levels at or below 1000 ppm, requiring large quantities of desiccant when used in the standard methods of the prior art. Because space within the standard enclosure is typically limited to minimize the area and/or depth of the enclosure, low capacity desiccants are typically not used, even when these desiccants have the desirable feature of fast water absorption. In addition, the minimum achievable humidity level of molecular sieve materials is a function of temperature within an enclosure: when using molecular sieve materials in the standard methods of the prior art, moisture absorbed, for example, at room temperature can be released into the enclosure or package during temperature cycling to higher temperature, for example, a temperature of 100° C.
Solid water-absorbing particles currently used within such packaged devices typically include 0.2 to 200 μm particle size powders of metal oxides, alkaline earth metal oxides, sulfates, metal halides, or perchlorates, i.e. materials having relatively low values of equilibrium minimum humidity and high moisture capacity. However, even when finely divided into powders of 0.2 to 200 μm particle size, such materials often chemically absorb moisture relatively slowly compared to the above-mentioned molecular sieve, silica gel, or Drierite materials. Such relatively slow reaction with water vapor can lead to a measurable degree of performance degradation if the rate of moisture permeating through the seal between the device and the cover exceeds the rate of water absorption by the desiccant.
Numerous publications describe methods and/or materials for controlling humidity levels within enclosed or encapsulated electronic devices. Kawami et al., in U.S. Pat. No. 5,882,761, has taught the use of a desiccant layer over the organic layers of an OLED display, between the substrate and the top seal. Kawami et al. teach the use of the following desiccants: alkali metal oxides, alkali earth metal oxides, sulfates, metal halides, and perchlorates. Such materials can be deposited in a predetermined shape by such techniques as vacuum vapor deposition, sputtering, or spin-coating. Boroson et al., in U.S. Pat. No. 6,226,890, disclose the use of a castable blend of the above desiccants with a suitable binder. However, many desiccating agents can be reactive toward the layers and electrodes of OLED devices, and a number of ways have been proposed to keep the desiccating agents from contacting the OLED components when used inside the same enclosure as the OLED components. Kawami et al., in the '761 patent, have taught that the drying agent is to be coated on the inside surface of an airtight container. Boroson et al., in the '890 patent, use the castable blend to coat the interior surface of an enclosure. The requirement that the desiccating agents of the prior art not be reactive toward the layers and electrodes of OLED devices and the requirement to keep the desiccating agents from contacting the OLED components limits the choices of possible desiccating agents.
The methods of Kawami et al., in U.S. Pat. No. 5,882,761 and Boroson et al., in U.S. Pat. No. 6,226,890 rely on the seal between the substrate and the enclosure to limit the rate of water permeation and require sufficient volume within the enclosure to hold enough desiccating agent to absorb moisture for the entire lifetime of the device. These limitations can require very wide seals, and thus wide borders beyond the display area, to prevent the rate of moisture permeation from exceeding the rate of moisture absorption by the desiccating agents. Such an arrangement can require large or deep enclosures to hold sufficient quantities of desiccating agent. For top emitting OLED devices in particular, this enclosure size requirement can be a significant issue since most desiccating agents are not transparent, and therefore cannot be located over the emitting areas of the OLED. For top emitting OLED devices, desiccating agents must typically be placed outside the display area, but still inside the enclosure, resulting in large borders beyond the display area. It is desirable to maintain small borders beyond the display area of OLED devices to minimize the size of a given device and to maximize the number of devices produced on a given mother glass substrate during manufacturing.
Tsuruoka et al., in U.S. Patent Application Publication 2003/0110981, have disclosed a series of transparent drying agents which operate by chemisorption and can be used in an OLED display. These are conceived as useful in OLED devices wherein one wishes to allow light emission through a desiccant layer. However, a desiccant—especially a chemisorption desiccant—is designed to change in the presence of moisture. Therefore, it is possible that the properties of the optical path of the device will change during the device lifetime, leading to potential visual changes in the display. This can limit the usefulness of this method.
Selection of solid water-absorbing particles and the method of applying selected particles to an inner portion of a device enclosure prior to sealing the device within or by the enclosure are governed by the type of device to be protected from moisture. For example, highly moisture-sensitive organic light-emitting devices or polymer light-emitting devices require the selection of particular solid water-absorbing particles and methods of application, since organic materials or organic layers are integral constituents of such devices. The presence of organic materials or layers can, for example, preclude the use of certain solvents or fluids in the application of fluid-dispersed solid water-absorbing particles to organic-based devices. Furthermore, a thermal treatment, if required, of a desiccant contained within a sealed device enclosure, needs to be tailored to the constraints imposed by thermal properties of the organic constituents or layers of the device. At any rate, release of solvent vapors during a thermal treatment of a desiccant disposed within a sealed device enclosure must be avoided or minimized if solvent vapors can adversely affect organic constituents of the device.
Shores, in U.S. Pat. Nos. 5,304,419, 5,401,536, and 5,591,379 discloses moisture gettering compositions and their use for electronic devices. However, many of the desiccants disclosed by Shores will not function effectively with highly moisture-sensitive devices at a humidity level lower than 1000 ppm. Similarly, binders, such as polyethylene disclosed by Shores, which have low moisture absorption rates compared to the absorption rate of the pure selected desiccants, would not function effectively to achieve and to maintain a humidity level below 1000 ppm during a projected operational lifetime of a highly moisture-sensitive device.
Deffeyes, U.S. Pat. No. 4,036,360 describes a desiccating material that is useful as a package insert or on the interior walls of packaging boxes for applications requiring only moderate moisture protection, such as film or cameras. The material includes a desiccant and a resin having a high moisture vapor transmission rate. The desiccants disclosed by Deffeyes are alumina, bauxite, calcium sulfate, clay, silica gel, and zeolite, but Deffeyes does not describe the particle size of any of the desiccants. None of these desiccants, other than zeolite, will function effectively with highly moisture-sensitive devices at a humidity level lower than 1000 ppm, and zeolite has the problem described above of low capacity at humidity levels lower than 1000 ppm. In addition the moisture vapor transmission rate requirement for the resin is not adequately defined since there is no reference to the thickness of the measured resins. A material that transmits 40 grams per 24 hrs per 100 in2 at a thickness of 1 mil would be very different than one that transmits 40 grams per 24 hrs per 100 in2 at a thickness of 100 mils. It is therefore not possible to determine if the moisture vapor transmission rates disclosed by Deffeyes are sufficient for highly moisture-sensitive devices.
Booe, U.S. Pat. No. 4,081,397, describes a composition used for stabilizing the electrical and electronic properties of electrical and electronic devices. The composition includes alkaline earth oxides in an elastomeric matrix. The desiccants disclosed by Booe are barium oxide, strontium oxide, and calcium oxide. Booe teaches the use of particle sizes less than 80 mesh (177 μm) to minimize the settling of oxides within the suspension. Booe does not teach the impact of particle size on desiccant performance. These desiccants will function effectively with highly moisture-sensitive devices at humidity levels lower than 1000 ppm; however, Booe claims the elastomeric matrix has the property of retarding the fluid absorption rate of the alkaline earth particles. In the examples, the water-absorption rate of the compositions is 5 to 10 times slower than the alkaline earth particles alone. This decrease in absorption rate is disclosed as a desirable feature that improves the handling of the highly reactive alkaline earth oxides. In highly moisture-sensitive devices, however, any decrease in the absorption rate of moisture will increase the likelihood of device degradation, and identification of resins that will increase the absorption rate of moisture would be highly desirable. For highly moisture-sensitive devices, therefore, it is important to determine the minimum allowable water vapor transmission rate of the binders used in combination with effective desiccant materials.
Organic light emitting diode (OLED) devices are moisture-sensitive electronic devices that can benefit from improved methods of providing desiccants and have a need for reduced moisture transmission rate into the device. Attempts at this in the art have been less than satisfactory. Kim et al. in U.S. Patent Application Publication 2003/0127976 A1 teach the use of two sealants surrounding an OLED device. While this can be a way to reduce the likelihood of sealant failure, it is no more effective at reducing moisture transmission rate into the device than would be a single wider sealant. Wang et al. in U.S. Patent Application Publication 2003/0122476 A1 show the use of two seals surrounding an OLED device with a desiccant between the two seals, but no desiccant inside the enclosure that contains the OLED device. This can reduce the moisture transmission rate into the device, but does not protect the OLED device from moisture sealed into the enclosure initially nor from moisture that permeates the inner seal. In addition, Wang et al. require the use of ribs that must be formed between the seals in order to hold the desiccant, adding complexity and expense to the fabrication process. Peng in U.S. Pat. No. 6,589,675 B2 also teaches the use of two seals with a desiccant between them. However, Peng requires the use of a separate sealing ring to hold the desiccant, adding extra steps and complexity to the fabrication process. Peng also fails to provide protection for the OLED devices from moisture that penetrates the interior seal. In addition, the methods of Wang et al. and Peng require wide borders beyond the display area to provide space for the two seals and desiccant.
Rogers, U.S. Pat. No. 6,081,071, describes an OLED device on a transparent substrate over which a cover is provided. The cover is attached to the OLED substrate using inner and outer concentric adhesive rings. A “desiccant and/or an inert fluorocarbon liquid” is provided between the adhesive rings and interior of the inner ring. Rogers does not disclose any particular desiccant materials other than metal salts such as CoCl2. As fluorocarbons, Rogers discloses using various commercially available Fluorinert® materials. Rogers clearly teaches that the desiccant is optional and one need only use the fluorocarbon material. Fluorocarbons are not desiccants—they do not bind water. Rather, the function of the fluorocarbon in Rogers is to act as a water barrier, i.e., provide some resistance to water transmission. This is functionally similar to what the adhesive rings do. Rogers fails to recognize the criticality of including an actual desiccant both between the adhesive rings and within the enclosure containing the OLED device. In this embodiment, the method of Rogers is similar to those of Wang et al. and Peng. With no desiccant within the enclosure containing the OLED device, the OLED device is not protected from moisture sealed initially within the enclosure, nor from moisture that permeates the inner adhesive ring. In addition, the method of Rogers, like those of Wang et al. and Peng, requires wide borders beyond the display area to provide space for the two seals and desiccant.
Boroson, U.S. Patent Application Publication 2006/0022592, describes a method for reducing moisture contamination in a top-emitting OLED device. A top-emitting electroluminescent (EL) unit is formed over the top surface of a substrate, wherein the EL unit produces light that is not emitted through the substrate. First and second protective covers are formed over the top and bottom substrate surfaces, respectively, thereby defining first and second chambers, respectively. A moisture-absorbing material is placed within the second chamber and communication between the first and second chambers is provided whereby moisture in the first or second chambers is absorbed by the moisture-absorbing material. Since the emitted light from the OLED device does not pass through the second chamber that contains the desiccant, the choice of desiccants for preserving the lifetime of top-emitting OLED displays is not limited by optical properties. In addition, the choice of desiccant materials is not limited to materials that remain solid after absorbing water since the desiccant is physically separated from the OLED devices. The method described allows for a top-emitting OLED display with a narrow seal and small borders, potentially allowing a top-emitting OLED display to be the same overall area as a bottom-emitting device of the same display area. In addition, locating the desiccant on the non-emitting side of the substrate allows for an OLED with a large capacity for moisture without increasing the display area to accommodate the desiccant. The method described, however, does not address the problem of high moisture permeation rates that can exceed the rate of moisture absorption by the desiccant. Because the first and second chambers are in direct vapor communication, the seal of the first chamber does not provide any additional resistance to moisture permeation beyond a traditional single seal. To decrease the rate of moisture permeation in this method would require the same wide borders as required in the method of Kawami et al. described above.
Therefore, there still remains the need to reduce moisture transmission rate into highly moisture-sensitive devices, such as OLED devices, in a way that does not add to the size of the border required beyond the display area of an OLED device, and also the need to protect these highly moisture sensitive devices from any moisture that penetrates the protective seals encapsulating these devices.